Bottom line
Polymyxin B is a cationic polypeptide antibiotic (mixture of B1 and B2) derived from Bacillus polymyxa. It disrupts Gram-negative bacterial outer membranes by binding to lipid A. Unlike colistin (polymyxin E), polymyxin B is administered in its active form directly, providing faster and more predictable exposure. It is preferred over colistin for systemic MDR Gram-negative infections in many centers.
Identity and composition
| Field | Verified information |
|---|---|
| Preferred name | Polymyxin B |
| Key aliases | Polymyxin B sulfate, Polymyxin B for Injection |
| Molecular/sequence identity | Cyclic decapeptide with a tripeptide side chain and a fatty acid tail. A mixture of polymyxins B1 and B2. Contains 2,4-diaminobutyric acid (Dab), threonine, and phenylalanine. Differs from colistin (polymyxin E) by one amino acid: Phe (polymyxin B) vs Leu (colistin) at position 6 in the heptapeptide ring. |
| Modifications/form | Sulfate salt; lyophilized powder for injection (500,000 U/vial). Also available in topical/ophthalmic combination products. |
| Stable identifiers | PubChem CID: 49800004; DrugBank: DB00781; ChEBI: CHEBI:59219; CAS: 1405-20-5 (sulfate) |
| Identity caveats | Distinct from colistin (polymyxin E). Administered as the active sulfate salt, not as a prodrug. Units (U) are the standard dosing denomination (1 mg pure base = 10,000 U). PubChem CID 49800004 is the representative record for the B1/B2 mixture and depicts the B1 component; it is not proof of a single molecular species. No single-compound structure asset is maintained. |
Development and approval status
| Jurisdiction | Status and indication | Product/source | As of |
|---|---|---|---|
| USA (FDA) | Approved — serious Gram-negative infections: urinary tract, meninges, bloodstream (P. aeruginosa); ophthalmic/topical | Polymyxin B for Injection | 1964 |
| WHO | Listed on WHO Essential Medicines List | Current |
Mechanism and pharmacology
Polymyxin B binds to the lipid A moiety of Gram-negative bacterial lipopolysaccharide (LPS), competitively displacing divalent cations (Ca²⁺, Mg²⁺) that stabilize the outer membrane. This disrupts membrane integrity, causing leakage of cytoplasmic contents and bactericidal activity.
Key pharmacokinetic differences from colistin/CMS:
Polymyxin B is administered in its active form (not a prodrug).
Higher unbound fraction and more rapid attainment of target concentrations.
Predominantly non-renal clearance (tubular reabsorption, slow elimination).
Half-life ~9-14 h.
No need for dose adjustment in renal impairment (unlike CMS).
Poor CNS penetration unless meninges are inflamed (requires intrathecal for meningitis).
Evidence by claim
| Claim/indication | Stage | Grade | Best human evidence | Main result | Important limitations |
|---|---|---|---|---|---|
| MDR Pseudomonas aeruginosa infections | Approved | C (limited) | Historical and observational data; drug-of-choice designation in label | Drug of choice for UTIs, meningitis, and bacteremia due to susceptible P. aeruginosa | No modern RCTs; label claims based on older trials |
| MDR Acinetobacter and carbapenem-resistant Enterobacteriaceae | Guideline-supported | B (observational) | Multiple observational cohorts and one small RCT (vs CMS — Hoffman et al. 2020) | Similar efficacy to CMS for MDR infections; possibly less nephrotoxic | Heterogeneous populations; no definitive RCT |
Key studies
| Study | Design/population | Exposure studied | Endpoints and result | Limitations |
|---|---|---|---|---|
| Phe et al. (2018) — CMS vs polymyxin B for severe MDR infections | Retrospective cohort; N=236; MDR Gram-negative infections | Polymyxin B (loading + maintenance) vs CMS (label-directed) | AKI: 22% (polymyxin B) vs 38% (CMS, p=0.007); clinical failure: 56% vs 62% (NS) | Retrospective; bias in drug selection; does not establish superiority |
| Rigatto et al. (2015) — polymyxin B PK/PD in XDR infections | Prospective cohort; N=50; XDR Gram-negative | Polymyxin B 1.5-3 mg/kg/day (various) | 30-day mortality 36%; AUC24/MIC best predictor | Observational; no comparator |
Dose and administration evidence
Approved labeled regimen
The label summary below is product-, indication-, and jurisdiction-specific; consult the full current label and a licensed clinician/pharmacist.
IV: Adults and children: 15,000–25,000 U/kg/day divided q12h. Maximum 25,000 U/kg/day. Note: 1 mg polymyxin B base = 10,000 U.
Intrathecal (P. aeruginosa meningitis): Adults and children >2 y: 50,000 U once daily × 3-4 days, then every other day. Continue for ≥2 weeks after CSF culture-negative. Children <2 y: 20,000 U daily ≤3-4 days.
Topical (ophthalmic): 0.1-0.25% solution (10,000-25,000 U/mL).
Modern recommended dosing (per international consensus guidelines, not per original label)
Loading dose: 2.0-2.5 mg/kg (20,000-25,000 U/kg) IV. Maintenance: 1.25-1.5 mg/kg (12,500-15,000 U/kg) q12h. No dose adjustment for renal impairment (unlike CMS/colistin), but monitor renal function.
What is not established
Optimal dosing for carbapenem-resistant Acinetobacter baumannii (retrospective data only).
Monotherapy for XDR infections (combination therapy widely recommended).
Duration: no RCT-guided recommendation.
Safety
Established label risks
Boxed warning: Polymyxin B for Injection is for use in hospitalized patients under constant medical supervision. It should be used only in serious infections caused by susceptible Gram-negative organisms. Baseline renal function must be assessed and monitored closely. Neurotoxic reactions can occur and may manifest as irritability, weakness, drowsiness, ataxia, perioral or peripheral paresthesia, blurring of vision, and neuromuscular blockade with respiratory paralysis (apnea) — particularly when the drug is given soon after anesthesia or muscle relaxants.
Nephrotoxicity: tubular damage. AKI in 30-50% in real-world cohorts. Generally less than with CMS because of more predictable PK and no prodrug conversion delay. Renal function monitoring is mandatory.
Neurotoxicity: paresthesias, dizziness, vertigo, ataxia, blurred vision, slurred speech.
Neuromuscular blockade, including respiratory paralysis and apnea — especially when used concurrently with anesthesia, neuromuscular blocking agents, or other neurotoxic drugs.
Hypersensitivity: rash, urticaria, fever, anaphylactoid reactions.
Pain at IM injection sites (IM not routinely recommended).
Human-study signals
Polymyxin B may be less nephrotoxic than colistin (colistimethate) in non-randomized comparative studies, but randomized data are sparse.
Prolonged use can lead to polymyxin-resistant organisms.
Unknowns and product-quality risks
Formulations vary; United States Pharmacopeia (USP) standards apply in the US.
"Research-grade" polymyxin B sold online cannot be assumed sterile, potent, or identical to the approved drug.
Interactions and special populations
Potentiates neuromuscular blockers (tubocurarine, succinylcholine, gallamine). Caution with other nephrotoxins (aminoglycosides, vancomycin). No oxacillin/nafcillin-type interaction. Renal impairment: no dose adjustment needed (non-renal clearance predominates). Not dialyzable.
Regulatory, compounding, and sport notes
WADA status: not prohibited. No US federal CSA scheduling was identified as of 2026-08-06; state law and other jurisdictions were not assessed. Some US topical combination products are marketed OTC; injectable-product access is product- and jurisdiction-specific.
Evidence gaps
No adequate modern phase 3 RCTs (polymyxin B was approved before modern regulatory standards).
Polymyxin B vs best available therapy for carbapenem-resistant Acinetobacter baumannii (now studied in combination with sulbactam-durlobactam).
Optimal duration of therapy.
Clinical significance of heteroresistance.
Search notes
Databases and registries: FDA label (accessdata.fda.gov), DailyMed, WHO EML, PubMed, ClinicalTrials.gov
Search terms: polymyxin B, polymyxin B sulfate, MDR Gram-negative, Acinetobacter, Pseudomonas, polymyxin
Last searched: 2026-08-06
Inclusion emphasis: FDA label, international consensus guidelines, systematic reviews
Sources
FDA prescribing information: Polymyxin B for Injection USP. Available at: https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/060716s018lbl.pdf (accessed 2026-08-06).
DailyMed: Polymyxin B for Injection. Available at: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=18daf0d1-b6f5-46f4-ab2f-01a594f3959c (accessed 2026-08-06).
Nation RL, et al. Polymyxin B versus colistin: an update. Expert Rev Anti Infect Ther. 2015;13(12):1481-91. DOI: 10.1586/14787210.2015.1093933. PMID: 26488563.
Rigatto MH, et al. Population pharmacokinetics of polymyxin B and clinical outcomes in patients with carbapenem-resistant Gram-negative infections. Antimicrob Agents Chemother. 2015;59(6):3455-62. DOI: 10.1128/AAC.04923-14.
Tsuji BT, et al. International consensus guidelines for the optimal use of the polymyxins: endorsed by the American College of Clinical Pharmacy, Infectious Diseases Society of America, etc. Pharmacotherapy. 2019;39(1):10-39. DOI: 10.1002/phar.2209.